Combustion engines convert the energy generated from fuel combustion into mechanical power. When the fuel source is carbon based, such as gasoline or diesel, the combustion is often incomplete resulting in the emission of hydrocarbons, nitrogen oxide, carbon monoxide, sulphur dioxide, ozone, and other chemical by-products. Because emissions from combustion engines contribute to air pollution that may be harmful to humans, animals, and the environment, many countries have regulations that restrict emissions. While technology has improved emissions over the decades, there is still a need for further improvement.